The present invention relates generally to the fabrication of ferroelectric memory devices and, more particularly, with a method to maximize the ferroelectric properties of Lead Germanium Oxide (PGO) thin films for ferroelectric memory applications, by orienting crystallization of the PGO film along the c-axis.
Ferroelectric thin films for use in electro-optics, pyroelectric, frequency agile electronics, and non-volatile memories have drawn much attention in recent years due to their bi-stable nature. Most of the studies on Ferroelectric Random Access Memories (FRAMs) have concentrated on the memory structure with one transistor and one capacitor. The capacitor is made by a thin ferroelectric film sandwiched between two conductive electrodes. The circuit configuration and read/write sequence of this type memory are similar to that of DRAMs except no data refreshing is necessary in FRAMs. The fatigue problem observed in ferroelectric capacitor, therefore, becomes one of the major obstacles that limit the realization of these memories on a commercial scale. Lead germanate (Pb5Ge3O11) thin films exhibit excellent fatigue properties, so these PGO thin films are very attractive materials for FRAM device applications.
The non-perovekite uniaxial ferroelectric Pb5Ge3O11, with polar direction parallel to the c-axis, belongs to the trigonal space group P3 at room temperature. This material transforms to the hexagonal, space group P6(=P3/m) paraelectric phase above the Curie temperature (Tc=178xc2x0 C.). Since this uniaxial ferroelectric Pb5Ge3O11 possesses only 180xc2x0 domains, there are no ferroelastic effects that tend to reorient domains though 90xc2x0 in order to relax the polarization. Interesting features of this material are the small dielectric constant and small remanent polarization, which are also suitable for ferroelectric non-volatile memory devices, especially for one transistor memory applications. Pb5Ge3O11 also has some potential for thermal detector applications because of its pyroelectric and dielectric characteristics.
Another area of research in ferroelectric non-volatile memory is the deposition ferroelectric thin film directly onto the gate area of FET, to form a ferroelectric-gate controlled FET. Ferroelectric-gate controlled devices such as metal-ferroelectric-silicon (MFS) FET have been studied as early as 1950s. Various modified MFSFET structures have been proposed. For example, Metal-Ferroelectric-Insulator-Silicon (MFIS) FET, Metal-Ferroelectric-Metal-Silicon (MFMS) FET, and Metal-Ferroelectric-Metal-Oxide-Silicon (MFMOS) FET. In response to the requirements of one transistor memory applications, ferroelectric materials should have low dielectric constant and small remanent polarization. Therefore, ferroelectric Pb5Ge3O11 thin films, which have a smaller remanent polarization of 4 xcexcC/cm2 and dielectric constant of about 50 in their bulk materials, have been sought.
The thin films of lead germanate were made in the past by thermal evaporation and flash evaporation (A. Mansingh and S. B. Krupanidhi, J. Appl. Phys. 51, 5408, 1980), dc reactive sputtering (H. Schmitt, H. E. Mueser, and R. Karthein, Ferroelectrics 56, 141, 1984), laser ablation (S. B. Krupanidhi, D. Roy. N. Maffei, and C. J. Peng, Proceedings of 3rd International Symp. on Integrated Ferroelectrics, 100, 1991), and sol-gel technique (J. J. Lee and S. K. Dey, Appl. Phys. Lett. 60, 2487, 1992).
Previously, single crystal Pb5Ge3O11 have been reported, with spontaneous polarization and coercive field of 4 xcexcC/cm2 and 14 kV/cm, respectively, in the direction along the c-axis. These c-axis oriented Pb5Ge3O11 thin films exhibit poor ferroelectric properties: lower polarization (2-3 xcexcC/cm2, higher coercive field (55-135 kV/cm ), and their hysteresis loops were not saturated and square. In order to switch the PGO ferroelectric domains, very high operation voltages are required, which precludes their use in the memory devices.
The PGO film of the present invention was developed meet the requirements of ferroelectric memory devices. The present invention concerns a pure c-axis oriented PGO thin films have a smaller Pr value, smaller dielectric constant and largest Ec value. Such a film is useful in making one transistor (1T) memory cells. In co-pending patent application Ser. No. 09/301,435, entitled xe2x80x9cMulti-Phase Lead Germanate Film and Deposition Methodxe2x80x9d, invented by Tingkai Li et al., filed on Apr. 28, 1999, a second phase of Pb3GeO5 is added to the Pb5Ge3O11, increasing grain sizes without an increase in c-axis orientation. The resultant film had increased Pr values and dielectric constants, and decreased Ec values. Such a film is useful in microelectromechanical systems (MEMS), high speed multichip module (MCM), DRAM, and FeRAM applications.
In co-pending patent application Ser. No. 09/302,272, entitled xe2x80x9cEpitaxially Grown Lead Germanate Film and Deposition Methodxe2x80x9d, invented by Tingkai Li et al., filed on Apr. 28, 1999, issued on Feb. 20, 2001 as U.S. Pat. No. 6,190,925, an appropriate content of the second phase Pb3GeO5 is added to Pb5Ge3O11, forming large grain sizes with extremely high c-axis orientation and completely epitaxial c-axis ferroelectric lead germanate film. As a result, high Pr and Ec values, as well as lower dielectric constant, is obtained. Such a film is useful in 1T, one transistor/one capacitor (1T/1C) FeRAM memory devices.
In co-pending patent application Ser. No. 09/301,434, entitled xe2x80x9cFerroelastic Lead Germanate Film and Deposition Methodxe2x80x9d, invented by Tingkai Li et al., filed on Apr. 28, 1999, a CVD) Pb3GeO5 film is funned having improved ferroelastic properties useful in making MEM and MOM devices. The above-mentioned co-pending patent application are incorporated herein by reference.
It would be advantageous if a single phase PGO film could be developed with ferroelectric properties sufficient for use in non-volatile memories.
It would be advantageous if the ferroelectric properties of a single phase polycrystalline PGO film could be enhanced by crystallographic alignment. Further, it would be advantageous if the crystalline PGO film could be aligned primarily along the c-axis.
It would be advantageous if a PGO film could be formed having a small, homogeneous, grain size for use in high density non-volatile ferroelectric memories.
Accordingly, in a lead germanium oxide (PGO) film, a method has been provided for forming a polycrystalline PGO film, having a c-axis orientation, on a IC film in a reactor chamber. The method comprising the steps of:
a) mixing [Pb(thd)2] and [Ge(ETO)4] to form a PGO mixture having a molar ratio in the range of approximately 5:3;
b) dissolving the mixture of Step a) with a solvent of tetrahydrofuran, isopropanol, and tetraglyme in a molar ratio of approximately 8:2:1, respectively, to form a precursor solution having a concentration of approximately 0.1 to 0.3 moles of PGO mixture per liter of solvent;
c) using a precursor vaporizer, heating the precursor solution to a temperature in the range of approximately 130 to 180 degrees C., creating a precursor gas;
c1) mixing the precursor gas in the chamber with an argon gas shroud flow in the range of approximately 1000 to 6000 standard cubic centimeters per minute (sccm), preheated to a temperature in the range of approximately 130 to 180 degrees C.;
c2) introducing an oxygen flow to the chamber in the range of approximately 500 to 3000 sccm;
d) heating the wafer to a temperature in the range of approximately 450 to 500 degrees C., to decompose the precursor gas formed in Step c) on the wafer;
e) forming a PGO film, including a first phase of Pb5Ge3O11 with a small, homogeneous, crystal grain size; and
f) forming a c-axis crystallographic orientation of approximately 70%, or more, in the Pb5Ge3O11 phase of the PGO film, and a grain size in the range of approximately 0.2 to 0.8 microns, whereby the ferroelectric properties of the PGO film are optimized.
In some aspects of the invention, Step f) includes the following sub-steps of:
f1) simultaneously with the deposition if the PGO film in Step e), orienting the polycrystalline structure of the PGO film in a primarily c-axis orientation; and
f2) following Step e), annealing the PGO film formed in Step e) at a temperature in the range of approximately 450 to 550 degrees C. in an atmosphere selected from the group of oxygen and oxygen with Pb atmospheres, whereby the c-axis orientation of the polycrystalline PGO film is enhanced;
In some aspects of the invention, a ferroelectric device is formed with the PGO film of in Step e), and includes further steps, following Step f2), of:
g) forming a conductive electrode underlying the PGO film formed in Step e); and
h) annealing the PGO film formed in Step e) at a temperature in the range of approximately 450 to 550 degrees C. in an atmosphere selected from the group of oxygen and oxygen with Pb atmospheres, whereby the interface between the PGO film, formed in Step e), and the electrode formed in Step g), is improved.
Steps f2) and h) include using a rapid thermal annealing (RTA) process to anneal the PGO film having a thermal ramp-up rate in the range of approximately 10 to 200 degrees C. per second, and a time duration of approximately 10 minutes.
A lead germanium oxide (PGO) film having improved ferroelectric properties is also provided. The PGO film comprises a first phase of polycrystalline Pb5Ge3O11, where Pb5Ge3O11 phase has primarily a c-axis crystallographic orientation. The c-axis orientation promotes ferroelectric film properties. In some aspects of the invention, approximately 70%, or more, of the Pb5Ge3O11 film has the c-axis orientation. The polycrystalline Pb5Ge3O11 film also includes crystal grains having a grain size in the range of approximately 0.2 to 1.5 microns.
A capacitor having ferroelectric properties is also provided. The capacitor comprises a first conductive electrode, a PGO film including a polycrystalline Pb5Ge3O11 phase with a primarily c-axis crystallographic orientation over the first electrode, and a second conductive electrode overlying the PGO film. The capacitor has a 2Pr of approximately 3.8 microcoulombs per centimeter squared (uC/cm2) and a 2Ec of approximately 93 kilovolts per centimeter (kV/cm) at an applied voltage of 8 volts.